Alkyl and aryl sulfonic acids and esters have been suggested as stabilizing agents for certain polylactams, see U.S. Pat. No. 3,878,173. Several compounds have been suggested as activators or promoters of the anionic (i.e., alkaline-catalyzed) polymerization of 2-pyrrolidone to polypyrrolidone, including sulfonyl azetidinone, see U.S. Pat. No. 3,185,677. The N-acyl compounds, such as that obtained from the reaction of acetic anhydride with 2-pyrrolidone, are a typical class of polymerization promoters, see U.S. Pat. Nos. 2,809,958 and 3,681,295. N-acyl compounds also serve as co-activators of polymerization in the carbon dioxide-activated anionic polymerization of pyrrolidone, see U.S. Pat. Nos. 3,721,652 and 3,681,295. That is, they serve to promote polymerization, i.e., further increase the rate of polymerization, under conditions of carbon dioxide-activated polymerization. Although in general the N-acyl co-activators, like other co-activators, increase the rate of polymerization, they do so at the cost of showing minimal improvement or actual lowering of the viscosity (molecular weight) of the polymer produced.